Histidyl-proline diketopiperazine is a metabolite of thyrotropin-releasing hormone. We have found this compound to have some hormone-like activities. A study of the fate of injection radioactive histidyl-proline diketopiperazine after injection into rats, was carried out. The distribution of the compount throughout the body wascharacterized. The rate of clearance of the peptide from the blood stream into the urine was evaluated. Binding of the diketopiperazine to membrane preparations was demonstrated.